The American Television Standards Committee (ATSC) transmission format for digital television (DTV) uses an 8 level vestigial sideband (8VSB) technique in which each successive 3 bit symbol is transmitted as one of 8 possible signal amplitudes. In a 4VSB system, each successive 2-bit symbol is transmitted as one of 4 possible signal amplitudes. In a 2VSB system, each successive 1-bit symbol is transmitted as one of 2 possible signal amplitudes. A 2VSB signal (or 4VSB signal) is more robust than an 8VSB signal because the distance between permissible signal levels is greater, making the transmitted signal more impervious to noise bursts and signal distortions.
It would be desirable to add a robust extension to the ATSC transmission format to enable the TV broadcasters to serve both the HDTV fixed receiver market and the portable market. Simultaneously, there has been a recent proposal within the ATSC to add “training packets” to the ATSC signal to enhance the receivability of the current DTV signal. The ATSC format was designed primarily for fixed reception and is not currently well optimized for robust reception. The only suggestion to date for a robust mode for the ATSC standard was the use of a 2VSB signaling mode during robust transmissions. Unfortunately, a 2VSB signaling mode is not backward compatible with the existing 8VSB format for a number of reasons. First of all, 2 level signaling would render the current generation of advanced demodulator IC's that utilize blind equalization techniques obsolete. When the ATSC format was originally adopted, it was believed that the training sequence, which occur every 24 milliseconds, would be sufficient for tracking both static and dynamic multi-path. It has been determined through extensive field-testing that the repetition rate of the training sequence is too low to track dynamic multi-path. The problem of tracking dynamic multi-path changes occurring in less than 24 milliseconds has been partially solved by a number of the newer generation of receivers by utilizing blind equalization to acquire the VSB signal. One particularly effective type of blind equalization is the Constant Modulus Algorithm (CMA) that uses a third order error function to effectively “open the eye” so that decision directed equalization can be used. The CMA error function used for VSB is a real only valued signal since the received symbols at the slicer are real only since the q-component is the Hilbert transform of the real part. The introduction of 2VSB symbols interspersed with 8VSB symbols would cause the CMA error function to be mismatched. The detailed cause of the mismatch is outlined below.
The symbol set for 8VSB is {−7, −5, −3, −1, 1, 3, 5, 7}. In order to make 2VSB signaling backward compatible when operating in a decision directed mode, the transmitted symbols should be bipolar and from the 8VSB set. A natural choice would be {+5, −5}, however, it can be shown that this chosen symbol set as well as any other bipolar set from the 8VSB set is incompatible with the 8VSB set itself when utilizing blind equalization such as CMA. The incompatibility arises since the constant modulus for the 2VSB symbols is different from the one needed for the 8VSB symbols.
The modulus for the 8VSB symbols is: E{X**4}/E{X**2} where X is the transmitted symbols and E is the expected value. The required modulus to drive the received symbols to the desired levels of {−7, −5, −3, −1, 3, 5, 7} so that decision directed equalization can be used is:((−7)4+(−5)4+(−3)4+(−1)4+(1)4+(3)4+(5)4+(7)4)/((−7)2+(−5)2+(−1)2+(1)2+(3)2+(5)2+(7)2)=37.However, the modulus for the 2VSB symbol set {−5, 5} is:((−5)4+(5)4)/((−5)2+(5)2)=25.And the modulus for the 2VSB symbol set {−7, 7} is((−7)4+(7)4)/((−7)2+(7)2)=49.
Therefore it can be seen that either form of 2VSB: {−5, 5} or {−7, 7} is incompatible with 8VSB signaling with respect to the modulus requirements for blind equalization. Therefore, if the 2VSB signaling format is used with existing (i.e., legacy) demodulator ICs that use the 8VSB modulus for blind equalization, the equalized symbol levels will be incompatible with the levels needed for decision directed mode. More specifically, if the 2VSB symbols {−5, 5} are interspersed with 8VSB symbols, the equalized received symbols will be greater in level than expected by legacy (i.e., existing) receivers, reflecting the fact that the expected value of the 2VSB symbols is lower that the 8VSB symbols on average. The blind equalizer then will compensate for this level mismatch by creating a new symbol set with an effective modulus of 37. Conversely, if the 2VSB symbols {−7, 7} are used, the equalized symbols will be lower in level than expected. The mismatch between CMA and decision directed symbol levels is a function of the number of 2VSB symbols injected into the 8VSB symbol stream. Also, the mismatch will lead to a failure to acquire the signal when there is severe multi-path and/or significant gaussian noise and the critical handoff from blind to decision directed is compromised.
The introduction of training packets to aid equalization reduces the payload capacity of the channel. Each 8VSB symbol carries 2 bits of information and 1 bit of redundancy introduced by the trellis code. This type of coding is referred to as 2/3 rate trellis coding. Symbols that are derived from known training packets contain 0 bits of information and 3 bits of redundancy. Two of the redundant bits come from the known training packet in the payload itself and 1 additional bit of redundancy from the trellis code. These types of symbols are referred to as 0/3 rate symbols. Since 0/3 rate symbols carry no information, they are simply overhead, and are to be avoided if at all possible.